Ex-Prime Minister Gordon Brown said: “There will be worldwide condemnation of a new video showing Boko Haram cruelly and barbarically using 200 kidnapped girls to bargain for the release of prisoners and exploiting innocent young girls for political purposes.”

Mr Brown, who in his role as UN envoy for global education met Nigeria’s president on Friday in the capital Abuja, added: “It is urgent that all religious leaders in every part of the world speak out against Boko Haram’s perverted and twisted version of Islam, which involves forced conversions and the sale of girls as sex slaves.”

Boko Haram: Leader Abubakar Shekau. (Image: Getty)

It is understood that the majority of the abducted girls are Christians.

Speaking in a mixture of the Hausa and Arabic languages, Shekau suggested in the video that his swap deal only referred to girls who had not become Muslims.

He added: “We will never release them until after you release our brethren. Here I mean those girls who have not submitted [converted to Islam].”

The terror boss said the children who have turned to Islam have become “sisters of the militants”.

It raises the prospect that converts – who have potentially been forced by the thugs into following Islam – may never be released.

The 17-minute video shows more than half of the roughly 200 girls who remain missing.

Analysts believe the other captives have been split into smaller groups and moved to avoid detection.

The footage emerged after Kashim Shettima, the governor of Borno state where the girls were seized, said he knew where some are being held.

Mr Shettima said he had been told about sightings of the girls and had passed on this information to the military.

Nigeria has deployed two divisions of troops in the search, which could mean as many as 30,000 soldiers.

Britain has sent spooks and police experts to help the hunt, and David Cameron did not rule out dispatching our troops if asked.

The PM’s spokesman said: “The first thing I would say about this kind of report is it underlines the horror and barbarity of the actions in the kidnapping of the girls.

Wait: Mothers of the missing Chibok schoolgirls (Image: Getty)

“It underlines the importance of working with and in support of the Nigerian people in trying to bring about the release of the kidnapped girls.

"One dreads to think about the ordeal they are undergoing.

“One of the advantages of having a team on the ground is it puts us in a position to consider what more we can do.”

The crisis appeared to worsen as Boko Haram reportedly blew up two bridges to further destabilise northern Nigeria and slow down the search.

A French official said Nigerian president Goodluck Jonathan had agreed to attend a summit in Paris to focus on the Boko Haram network.

Nigerian officials said 276 girls, aged from 12 to 15, were abducted from the school in Chibok, Borno, on April 14.

Some escaped, but around 200 are still missing.

Boko Haram slaughtered 300 civilians in the Borno town of Gamboru Ngala last week.